Method and a system for controlling vehicle speed

ABSTRACT

A device and a method for controlling vehicle speed in a vehicle equipped with brake cruise control when the vehicle is travelling downhill are provided. The method involves driving a vehicle downhill with the brake set speed set to a first brake set speed; detecting a current vehicle speed; automatically applying a brake torque using at least an auxiliary brake to maintain the first brake set speed; and detecting a manual application of a vehicle service brake, in order to decrease vehicle speed. If a control unit detects that the driver is applying the service brake, then the control unit is automatically arranged to set the brake set speed to a second brake set speed that is lower than the first brake set speed; and to apply a brake torque using at least the auxiliary brake if a detected current vehicle speed exceeds the second brake set speed.

BACKGROUND AND SUMMARY

The invention relates to a method and a system for controlling vehiclespeed in a vehicle equipped with brake cruise control when the vehicleis travelling downhill. The invention can be applied in heavy-dutyvehicles, such as trucks, buses and construction equipment. Although theinvention will be described with respect to a heavy-duty vehicle, theinvention is not restricted to this particular vehicle, but may also beused in other vehicles such as articulated haulers, wheel loaders, andother working machines or automobiles.

Driver assistance systems for automatically controlling the speed of avehicle, often termed cruise control systems, allow a driver to specifya selectable driving speed via actuating means. The set speed is thenmaintained independently of the operation of an accelerator pedal of thevehicle. The speed can be set by the driver and can also be retainedindependently of the driving situation, such as uphill or downhilldriving. The vehicle will then use suitable means such as auxiliarybrakes or service brakes to maintain the set speed.

DE 10 2006 051 179 A1 (corresponding to U.S. Patent App. Pub.US2007/0099579) relates to a method for automatic downward velocitycontrol, commonly known as brake cruise control, which is capable ofabsorbing energy induced by the inclination automatically. In this waythe driver must no longer be active to maintain a substantially constanttraveling speed. The downward velocity control is triggered when theaccelerator pedal is released, and the vehicle begins to acceleratedownhill. The vehicle speed is controlled towards target vehicle speed,which is a desired predetermined constant vehicle speed set by thedriver.

When travelling downhill in a vehicle with brake cruise control, it isnot always possible to maintain the brake set speed by using auxiliarybrakes only. Reducing the vehicle speed and/or manually resetting thebrake set speed is often necessary to avoid over-using the servicebrakes in long or steep downhill conditions. Extended periods of servicebrake actuation can cause the brakes to overheat, which can cause thebrakes to wear at an increased rate and may even damage the brakes.

In a conventional vehicle this has to be done by the driver by manuallyadjusting the brake set speed and pressing the brake pedal to actuatethe service brakes to reduce the vehicle speed down to the desiredspeed. At the end of the downhill road section, the brake set speed hasto be manually adjusted by the driver back to the originally set value.

It is desirable to provide a method and a system for controlling vehiclespeed in a vehicle equipped with brake cruise control which eliminatesthe above problems. The invention allows, according to an aspectthereof, automatic control of the speed of a vehicle requiring a minimumof intervention and providing a high level of comfort for the driver.

It is desirable to provide a method and a system for controlling vehiclespeed in a vehicle equipped with brake cruise control.

In this context, the term “service brake” is used to denote the normal,or main brakes used in heavy-duty vehicles, usually operated by thedriver depressing a service brake pedal. The service brakes are often,but not necessarily, air brakes operated by compressed air controllingfriction brakes provided on each wheel to be braked on the vehicle. Theterm “auxiliary brakes” is used to denote additional means for assistingbraking of the vehicle. Auxiliary brakes are used in addition to theservice brakes and allow for prolonged brake application withoutrequiring assistance from the service brakes. In this way, wear andunnecessary heating of the service brakes can be avoided. Such auxiliarybrakes can comprise an exhaust valve, for activating an exhaust brake,engine brake valves, for activating a compression release engine brake,a fluid retarder, for generating a braking force on a propeller shaft,or means for driving a compressor or generator to generate a brakingforce. For electric and hybrid electric vehicles the auxiliary brake cancomprise a generator, a motor-generator or other suitable means.

The invention, according to an aspect thereof is applicable to vehiclesprovided with a brake cruise control system. According to an aspect ofthe invention the system can be used for vehicles provided with internalcombustion engines, as well as for electric and hybrid electricvehicles. This cruise control system can be enabled by the driver priorto or during downhill travel. When the actual, or current vehicle speedexceeds a brake set speed set by the driver-, an auxiliary brake, suchas an engine brake, a motor-generator, or similar, will engage tomaintain that speed within the limits of the auxiliary brake.

According to a first aspect of the invention, a method for controllingvehicle speed in a vehicle equipped with brake cruise control when thevehicle is travelling downhill is provided. The method comprises thesteps of;

-   -   driving a vehicle downhill with the brake set speed set to a        first brake set speed;    -   detecting a current vehicle speed;    -   automatically applying a brake torque using at least an        auxiliary brake to maintain the first brake set speed;    -   detecting a manual application of a vehicle service brake, in        order to decrease the vehicle speed;

In this context, the steps of detecting a current vehicle speed andautomatically applying a brake torque using at least an auxiliary braketo maintain the first brake set speed are actions performed by the brakecruise control without input from the driver.

If a control unit provided in the vehicle detects manual application ofthe service brake, i.e the driver is applying the service brake, thenthe control unit is automatically arranged to perform the steps of:

-   -   setting the brake set speed to a second brake set speed that is        lower than the first brake set speed; and    -   applying a brake torque using at least the auxiliary brake if a        detected current vehicle speed exceeds the second brake set        speed.

An advantage of this method is that the driver is not required toperform any action, apart from briefly applying the brake pedal, toreduce the speed of the vehicle to a lower brake set speed. Noadditional switches or controls need to be actuated and no new set speedhas to be selected by the driver. A reduction of the manual actionsrequired for this purpose allows the driver to concentrate on the roadand improves the driving comfort.

According to a first alternative example, the control unit isautomatically arranged to set the second brake set speed at a levelwhere the second brake set speed can be maintained by the auxiliarybrake.

According to a second alternative example, the control unit isautomatically arranged to set the second brake set speed at the vehiclespeed detected when the driver released the service brakes.

The method is arranged to repeat the above steps until it is detectedthat the downhill condition has ended, i.e. the slope is levelling outand normal cruise control can be resumed. Hence, when travellingdownhill with the brake set speed to the second brake set speed it ispossible to perform further speed reductions if required. Should thecontrol unit detect a further manual application the service brake, forinstance caused by a steeper than expected slope, then the brake setspeed is set to a third brake set speed that is lower than the secondbrake set speed.

When the control unit detects that the downhill condition has ended thenthe brake cruise control brake set speed is restored to the first brakeset speed and one or more upshifts are performed. According to a firstalternative example this step can be performed automatically in responseto one or more signals transmitted to the control unit by an inclinationsensor, an accelerometer, an on-board topographic database, aGPS-receiver, or a similar suitable device.

According to a second alternative example the brake set speed isrestored to the first brake set speed when the driver issues a resumecommand. Such a resume command can involve the driver actuating anaccelerator means or a resume switch on the dashboard, or on or adjacentthe steering wheel. The resume command is detected by the control unit,which restores the first brake set speed

An advantage of the latter method steps is that the driver is notrequired to perform any action, or alternatively a single actioninvolving a brief application of the accelerator pedal or actuating oneswitch, to resume the first brake set speed of the vehicle. Noadditional switches or controls need to be actuated and no new/earlierset speed has to be selected by the driver. This further reduces themanual actions required, which allows the driver to concentrate on theroad and improves the driving comfort.

According to a second aspect of the invention, a vehicle speed controlsystem in a vehicle equipped with brake cruise control when the vehicleis travelling downhill is provided. The system comprises a sensorarranged to detect a current vehicle speed; a service brake;

at least one auxiliary brake; and a control unit having a userselectable input for receiving a brake set speed, said control unitbeing arranged to compare the current vehicle speed with a first brakeset speed and control the auxiliary brakes to maintain the first brakeset speed. The first brake set speed can be set once by the driver andis then stored in the control unit for future use. Alternatively, thesystem can prompt the driver for an input, e.g. by displaying thecurrently stored set speed value, and if no input is provided by thedriver then the stored first brake set speed is used. The speed sensorcan be a separate sensor provided for this purpose, a sensor formingpart of an ABS arrangement in the service brake, or a virtual sensorwhere the vehicle speed is calculated from suitable engine andtransmission parameters. The control unit is arranged to detect if theservice brake is applied to retard the vehicle speed. When a manualapplication of the service brakes is detected, the control unit isautomatically arranged to set the brake set speed to a second brake setspeed that is lower than the first brake set speed. The control unit issubsequently arranged to apply a brake torque using at least theauxiliary brake if the current vehicle speed exceeds the second brakeset speed. In this way, the auxiliary brake is used as the primarysource of brake torque. Only when the auxiliary brake or brakes cannotmaintain the second brake set speed will the service brake be actuatedby the control unit. Automatic actuation of the service brakes shouldonly be performed during short periods of time of intermittently thedriver may be notified if prolonged automatic application of the servicebrakes is carried out.

According to a first alternative example the control unit is arranged toset the second brake set speed at a level where the brake set speed canbe maintained by the auxiliary brake or brakes.

According to a second alternative example the control unit is arrangedto set the second brake set speed at the vehicle speed detected when thedriver released the service brakes.

When the control unit detects that the downhill condition has ended thenthe brake cruise control brake set speed is restored to the first brakeset speed. According to a first alternative example the control unit isarranged to restore the brake set speed to the first brake set speedwhen the control unit has detected that the downhill condition hasended. The control unit is arranged to perform this action in responseto one or more signals transmitted to the control unit by an inclinationsensor, an accelerometer, an on-board topographic database, aGPS-receiver, or a similar suitable device.

According to a second alternative example the control unit is arrangedto restore the brake set speed to the first brake set speed when thedriver issues a resume command. Such a resume command can involve thedriver actuating an accelerator means or a resume switch on thedashboard, or on or adjacent the steering wheel.

According to a third aspect of the invention, a computer program isprovided comprising program code means for performing theabove-mentioned method steps for controlling vehicle speed in a vehicleequipped with brake cruise control when said program is run on acomputer. The invention further relates, according to an aspect thereof,to a computer readable medium carrying a computer program comprisingprogram code means for performing the above-mentioned method steps forcontrolling vehicle speed in a vehicle equipped with brake cruisecontrol when said program product is run on a computer. Finally, theinvention further relates, according to an aspect thereof, to a controlunit for controlling vehicle speed in a vehicle equipped with brakecruise control, the control unit being configured to perform the stepsof the above-mentioned method.

By the provision of a method and a system which involve automaticcontrol of a brake cruise control system, the advantage is that thedriver can be relieved of a number of the manual actions by transferringcontrol of the brake cruise control system to a control unit. With thebrake cruise control system active, the driver merely needs to apply theservice brakes to indicate that a speed reduction is desired. When theservice brakes are released, the control unit will automatically set thebrake set speed to a lower value. This lower value is retained to theend of the downhill road section, or until the driver indicates afurther speed reduction. At the end of a downhill section the controlunit can be arranged to automatically reset the brake set speed to theinitial brake set speed. Alternatively, the driver can indicate that theinitial brake set speed is to be reset by issuing a resume command, forinstance by actuating a resume switch or by depressing an acceleratorpedal. In this way the number of manual actions required by the drivercan be reduced to an absolute minimum, allowing the driver toconcentrate on the road and improving the comfort for the driver.

Further advantages and advantageous features of the invention aredisclosed in the following description and in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

With reference to the appended drawings, below follows a more detaileddescription of embodiments of the invention cited as examples. In thedrawings:

FIG. 1 shows a schematic vehicle provided with a vehicle speed controlsystem according to one embodiment of the invention;

FIG. 2 shows a schematic diagram illustrating an example of speedvariation for a vehicle provided with a brake cruise control systemaccording to the invention;

FIG. 3A shows a schematic flow chart for performing the method accordingto the invention;

FIG. 3B shows an alternative schematic flow chart for performing themethod according to the invention; and

FIG. 4 shows the invention applied on a computer arrangement.

DETAILED DESCRIPTION

FIG. 1 shows a schematic vehicle provided with a vehicle speed controlsystem according to one embodiment of the invention. FIG. 1 shows avehicle 100, such as a tractor of a tractor semi-trailer vehicle, havingan electronically controlled internal combustion engine 110 coupled to atransmission 120 via a clutch mechanism 121. It should be noted that avehicle as shown in FIG. 1 only represents one of the possibleapplications for the system and method of the present invention. Thepresent invention can be implemented in any type of commercial orindustrial vehicle comprising a vehicle speed control system asdescribed herein.

The transmission 120 can be an automated mechanical transmission or analternative suitable transmission connected to an output shaft 122coupled to a driven axle 123 driving a pair of driven wheels 124. Thevehicle 100 includes at least two axles such as a steerable axle 127 andat least one rear driven axle 123. FIG. 1 shows a rear driven axle 123and a trailing axle 125 with associated wheels 124, 126, as well as asteerable axle 127 with associated wheels 128. Each axle 123, 125, 127supports corresponding wheels 124, 126, 128 having service brakecomponents 131, 132, 133 which can be manually or automatically actuateddepending upon the particular application and operating conditions. Forexample, a vehicle equipped with an anti-lock braking system (ABS) canassume automatic control of braking under appropriate conditions such aswhen the vehicle is braking and the system detects a sufficient slipdifferential among one or more wheels, or when the vehicle travelsdownhill under the control of a cruise control system. The service brakecomponents 131, 132, 133 include wheel speed sensors and electronicallycontrolled pressure valves (not shown) to effect control of the vehiclebraking system. The service brake components 131, 132, 133 are connectedto a central electronic control unit (ECU) 140 by means of suitablewiring, as schematically indicated by the input 134 and output 135 inthe figure. A wheel speed sensor 136 is schematically indicated inFIG. 1. The output signal from one or more such sensors can be used forcalculating the vehicle speed.

The vehicle 100 includes conventional operator controls such as anaccelerator pedal 141, a brake pedal 142 and an optional clutch pedal143 (for manual transmissions), as well as an operator interface, suchas dashboard control console (not shown). The dashboard control consolecan include any one of a number of output devices, such as lights,displays, buzzers, gages, and the like, and various input devices, suchas switches, push buttons, potentiometers, or the like. Examples ofinput devices can be controllers 144, 145 for setting and/or restoring abrake cruise control set speed in in a brake cruise control system. Thecontroller 144 for setting the brake cruise control set speed and thecontroller 145 for restoring the brake cruise control set speed are bothconnected to the ECU 140.

The vehicle control system includes the central ECU 140 which isconnected to electronic control modules 146, 147 for the engine 110 andthe transmission 120, respectively. The ECU 140 also communicates with avariety of sensors via inputs 148 and with numerous actuators viaoutputs 149. Sensors (not shown) may include a steering angle sensor,wheel speed sensors (included in service brake components 131, 132,133), an electronic accelerator pedal sensor, a brake pedal sensor orswitch, a clutch control/sensor, an output speed sensor, asensor/accelerometer 151 for indicating vehicle inclination, at leastlongitudinal inclination, and sensors for various engine parameters 152.

During downhill travel with the brake cruise control activated the ECU140 monitors information from the sensors and evaluates their input todetermine parameters such as current engine speed and torque, brakepedal position, accelerator position, road speed, cruise control status,and cruise control brake set speed, cruise control switches, clutchpedal position, among many others.

Actuators may include a shift actuator for automatically effecting agear shift within transmission 120, electronically controlled pressurevalves (included in service brake components 131, 132, 133), and one ormore auxiliary brakes, such as an engine retarder 111. An engineretarder is a device utilized to supplement the service brakes 131, 132,133 when descending long grades and to prolong service brake life inhigh-frequency start and stop operation. Retarders may be categorized asengine brakes, exhaust brakes, hydraulic retarders and electricretarders. One example of an engine retarder is an engine brake such asthe well-known “Jake brake”, which converts a power producing dieselengine into a power absorbing air compressor. This is achieved byshutting off the fuel and hydraulically opening the exhaust valve as thepiston approaches top dead center during the compression stroke. Manyengine manufacturers disable the engine brake when cruise control isengaged. However, the present invention can utilize the engine retarderwhen the brake cruise control is engaged to enhance vehicledeceleration. This is accomplished by directly controlling the engineretarder during downhill travel.

The ECU 140 contains logic rules, or algorithms implemented in a varietyof combinations of hardware circuitry components and programmedmicroprocessors to effect control of various vehicle systems andsubsystems. Cruise control functions can be incorporated as a cruisecontrol block within the ECU 140 which block represents the logic rulesnecessary to implement brake cruise control functions. The operation ofthe system will be described in further detail below.

FIG. 2 shows a schematic diagram illustrating an example of speedvariation for a vehicle provided with a brake cruise control systemaccording to the invention. As indicated in the figure, a vehicle 200equipped with brake cruise control is travelling along an initial roadsection X0 and is approaching a downhill slope. At a first point A thevehicle 200 is travelling at a speed that is equal to a first brake setspeed vB1. During a first downhill section X1 the vehicle will begin toaccelerate and the brake cruise control system, being arranged tomonitor the current vehicle speed, will attempt to reduce the speed tomaintain the first brake set speed vB1. The speed reduction is initiallycarried out by means of at least one auxiliary brake, preferably anengine retarder. Provided that the gradient of the downhill slope is notexcessive, the vehicle speed can be maintained below a predeterminedoverspeed vLIM, as indicated by the dashed curve in FIG. 2. However, atgreater gradients the overspeed vLIM can be exceeded as indicated by thefull curve in FIG. 2. In the latter case, the brake cruise controlsystem can automatically actuate the service brake to assist theauxiliary brake in limiting the speed. Actuation of the service brakesunder such conditions can trigger a warning to the driver, as extendeduse of the service brakes during steep or long downhill slopes isundesirable.

At a second point B, the driver intervenes to reduce the speed by manualactuation of the service brakes, e.g. by depressing the brake pedal.When the central control unit (ECU) provided in the vehicle (see FIG. 1)detects manual application of the service brake, then the ECU isautomatically arranged to perform a number of steps to reduce the speed,which steps do not require further actions from the driver.

In a first step, the ECU sets the brake set speed to a second brake setspeed vB2 that is lower than the first brake set speed vB1. The secondbrake set speed vB2 is selected by the ECU a level where the lower speedcan be maintained by the auxiliary brakes only. Alternatively, the ECUis arranged to set the second brake set speed vB2 at the vehicle speeddetected when the driver releases the service brakes. The action ofsetting a lower brake set speed can trigger an automated response from,for instance, the service brakes which can perform a brief, morepronounced vehicle retardation, or the transmission which can perform adown-change to a lower gear. The response to the lowering of the brakeset speed can be dependant of the currently detected road gradient. InFIG. 2, the vehicle is retarded from point B towards point C over asecond downhill section X2 as the speed is reduced towards the secondbrake set speed vB2.

In a second step, the ECU will apply a brake torque using at least theauxiliary brake, preferably only the auxiliary brake, if a detectedcurrent vehicle speed exceeds the second brake set speed vB2. In FIG. 2this is illustrated by point C, where the ECU determines that theauxiliary brake can be used for retarding the vehicle the vehicletowards and to maintain the second brake set speed vB2 over a thirddownhill section X3.

Under certain conditions where the gradient is greater than thestandardized norm for public roads, the above steps can be repeated ifthe driver reapplies the service brakes to reduce the speed further.

At point D the control unit detects that the downhill condition hasended. According to a first alternative example the ECU is then arrangedto restore the brake cruise control brake set speed to the first brakeset speed vB1 wherein the vehicle speed is increased to the first brakeset speed vB1 by the cruise control system over a fourth road sectionX4. The control unit is arranged to perform this action in response toone or more signals transmitted to the control unit by an inclinationsensor, an accelerometer, an on-board topographic database, aOPS-receiver, or a similar suitable device. One or more of thesesignals, in combination with a detected absence of auxiliary brakeactuation, can be used to detect that the downhill condition has ended.

According to a second alternative example the ECU is then arranged torestore the brake cruise control brake set speed to the first brake setspeed vB1 when the driver issues a resume command. Such a manual resumecommand can involve the driver actuating an accelerator means or aresume switch on the dashboard, or on or adjacent the steering wheel(see FIG. 1).

FIG. 3A shows a schematic flow chart for performing the method accordingto the invention in a vehicle equipped with a brake cruise controlsystem. As can be seen from the figure, the process is started in afirst step 311, where after a first brake set speed is set by the driverin a second step 312. In a third step 313 a downhill condition isdetected, causing the brake cruise control system to control the vehiclespeed in order to maintain the first brake set speed in a fourth step314. In this step, a number of parameters such as the vehicle speed androad gradient are monitored and at least the auxiliary brakes areapplied if the speed exceeds the first brake set speed. If no downhillcondition is detected in the third step 313 then the process is ended ina final step 317.

If the downhill condition continues, then an additional check isperformed in a fifth step 315 to detect if the service brakes have beenmanually actuated or not. If the service brakes have not been actuatedby the driver, then the process returns to the third step 313 todetermine if the downhill condition continues. The process then proceedsto the fourth step 314 to continue performing brake cruise control formaintaining the first brake set speed. On the other hand, if the servicebrakes have been actuated by the driver, then the process continues tosixth step 316, where the brake set speed to a second brake set speedthat is lower than the first brake set speed. The second brake set speedis set automatically by the brake cruise control system and can beselected from a stored table or matrix, and/or depending on parameterssuch as the current road gradient. Alternatively, the second brake setspeed can be set to the speed detected when the driver releases theservice brakes. The process then returns to the third step 313 to checkif a downhill condition exists. If this is the case then it proceeds tothe fourth step 314 to continue performing brake cruise control tomaintain the lower second brake set speed. As the set brake speed is nowlowered, the brake cruise control system can be required to perform adownshift and/or additional actions to reduce the vehicle speed to thenew set brake speed. The process then resumes as described above.

The process as described above is continued as long as a downhillcondition is detected in the third step 313. When the brake cruisecontrol system detects that the downhill condition has ended, then theprocess leaves the control loop and proceeds from the third step 313 toa final step 317. The end of a downhill condition is preferably detectedautomatically by the brake cruise control system, e.g. by monitoring theroad gradient. Alternatively, the end of a downhill condition isdetected when the driver actuates the accelerator or a switch providedfor this purpose. The process is ended in the final step 317 and revertsto regularly monitoring the detected parameters, only to be resumed if asubsequent downhill condition is detected. If the brake set speed hasbeen changed during a previous downhill section then it is automaticallyreset to the first set brake speed prior to a subsequent downhillsection.

FIG. 3B shows an alternative schematic flow chart for performing themethod according to the invention in a vehicle equipped with a brakecruise control system. As can be seen from the figure, the process isstarted in a first step 321, where after a first brake set speed can beset by the driver in a second step 322. The first brake set speed can beset once by the driver and is then stored in the control unit for futureuse. Alternatively, the system can prompt the driver for an input, e.g.by displaying the currently stored set speed value, and if no input isprovided by the driver then the stored first brake set speed is used. Ina third step 323 a downhill condition is detected, causing the brakecruise control system to control the vehicle speed in order to maintainthe first brake set speed in a fourth step 324. In this step, a numberof parameters such as the vehicle speed and road gradient are monitoredand at least the auxiliary brakes are applied if the speed exceeds thefirst brake set speed.

If the downhill condition continues, then an additional check isperformed in a fifth step 325 to detect if the service brakes have beenmanually actuated or not. If the service brakes have not been actuatedby the driver, then the process returns to the third step 323 todetermine if the downhill condition continues. The process then proceedsto the fourth step 324 to continue performing brake cruise control formaintaining the first brake set speed. On the other hand, if the servicebrakes have been actuated by the driver, then the process continues tosixth step 326, where the brake set speed to a second brake set speedthat is lower than the first brake set speed. The second brake set speedis set automatically by the brake cruise control system and can beselected from a stored table or matrix, and/or depending on parameterssuch as the current road gradient. Alternatively, the second brake setspeed can be set to the speed detected when the driver releases theservice brakes. The process then returns to the fourth step 324, via thethird step 323, to continue performing brake cruise control to maintainthe lower second brake set speed. As the set brake speed is now lowered,the brake cruise control system can be required to perform a downshiftand/or additional actions to reduce the vehicle speed to the new setbrake speed. The process then resumes as described above.

The process as described above is continued as long as a downhillcondition is detected in the third step 323. When the brake cruisecontrol system detects that the downhill condition has ended, then theprocess leaves the control loop and proceeds from the third step 323 toa seventh step 327. The end of a downhill condition is preferablydetected automatically by the brake cruise control system, e.g. bymonitoring the road gradient. Alternatively, the end of a downhillcondition is detected when the driver actuates the accelerator or aswitch provided for this purpose. In the seventh step 327 it isdetermined if the set brake speed has been changed or not. If the setbrake speed has not been changed then no action is required, and if thebrake set speed has been changed then it is reset to the first set brakespeed.

The process is ended in a final step 328 and reverts to monitoring thedetected parameters, only to be resumed if a subsequent downhillcondition is detected. If the brake set speed has been changed during aprevious downhill section then it is automatically reset to the firstset brake speed prior to a subsequent downhill section.

The present invention also relates to a computer program, computerprogram product and a storage medium for a computer all to be used witha computer for executing the method as described in any one of the aboveexamples.

FIG. 4 shows an apparatus 400 according to one embodiment of theinvention, comprising a non-volatile memory 420, a processor 410 and aread and write memory 460. The memory 420 has a first memory part 430,in which a computer program for controlling the apparatus 400 is stored.The computer program in the memory part 430 for controlling theapparatus 400 can be an operating system.

The apparatus 400 can be enclosed in, for example, a control unit, suchas the control unit 45. The data-processing unit 410 can comprise, forexample, a microcomputer. The memory 420 also has a second memory part440, in which a program for controlling the vehicle speed control systemaccording to the invention is stored. In an alternative embodiment, theprogram for controlling the vehicle speed control system is stored in aseparate non-volatile storage medium 450 for data, such as, for example,a CD or an exchangeable semiconductor memory. The program can be storedin an executable form or in a compressed state.

When it is stated below that the data-processing unit 410 runs aspecific function, it should be clear that the data-processing unit 410is running a specific part of the program stored in the memory 440 or aspecific part of the program stored in the non-volatile storage medium420.

The data-processing unit 410 is tailored for communication with thestorage memory 420 through a data bus 414. The data-processing unit 410is also tailored for communication with the memory 420 through a databus 412. In addition, the data-processing unit 410 is tailored forcommunication with the memory 460 through a data bus 411. Thedata-processing unit 410 is also tailored for communication with a dataport 490 by the use of a data bus 415.

It is to be understood that the present invention is not limited to theembodiments described above and illustrated in the drawings; rather, theskilled person will recognize that many changes and modifications may bemade within the scope of the appended claims.

The invention claimed is:
 1. Method for controlling vehicle speed in avehicle equipped with brake cruise control when the vehicle istravelling downhill, which method comprises the steps of: driving avehicle downhill with the brake set speed set to a first brake setspeed; automatically applying a brake torque using at least an auxiliarybrake to maintain a vehicle speed at the first brake set speed;detecting a manual application of a vehicle service brake whichdecreases the vehicle speed below the first brake set speed; and whenthe manual application of the service brake occurs, then automatically,via the control unit, performing the steps of: setting the brake setspeed to a second brake set speed that is lower than the first brake setspeed, and applying a brake torque using at least the auxiliary brakewhen a detected current vehicle speed exceeds the second brake setspeed; after the setting of the brake set speed to the second brake setspeed, determining that the detected current vehicle speed exceeds thesecond brake set speed with the auxiliary brake engaged; and in responseto the detected current vehicle speed exceeding the second brake setspeed, automatically applying, via the control unit, the service brakein conjunction with the auxiliary brake.
 2. Method according to claim 1,comprising setting the second brake set speed at the vehicle speeddetected when the driver released the service brakes.
 3. Methodaccording to claim 1, comprising restoring the brake cruise controlbrake set speed to the first brake set speed when the control unitdetects that the downhill condition has ended.
 4. Method according toclaim 1, comprising restoring the brake cruise control brake set speedto the first brake set speed when the driver issues a resume command. 5.Method according to claim 4, comprising restoring the brake set speed tothe first brake set speed when the driver actuates an accelerator means.6. Method according to claim 4, comprising restoring the brake cruisecontrol set speed to the first brake set speed when the driver actuatesa resume switch.
 7. Vehicle speed control system in a vehicle equippedwith brake cruise control when the vehicle is travelling downhill, whichsystem comprises: a sensor arranged to detect a current vehicle speed; aservice brake; at least one auxiliary brake; and a control unit having auser selectable input for receiving a brake set speed, the control unitbeing arranged to: compare the current vehicle speed with a first brakeset speed and control the auxiliary brakes to maintain the first brakeset speed; detect if the service brake is manually applied to retard thevehicle speed; and when a manual application of the service brakes isdetected, automatically: setting the brake set speed to a second brakeset speed that is lower than the first brake set speed, and applying abrake torque using at least the auxiliary brake when the current vehiclespeed exceeds the second brake set speed; after the setting of the brakeset speed to the second brake set speed, determining that the detectedcurrent vehicle speed exceeds the second brake set speed with theauxiliary brake engaged; and in response to the detected current vehiclespeed exceeding the second brake set speed, automatically applying, viathe control unit, the service brake in conjunction with the auxiliarybrake.
 8. Vehicle speed control system according to claim 7, wherein thecontrol unit is arranged to set the second brake set speed at thevehicle speed detected when the driver released the service brakes. 9.Vehicle speed control system according to claim 7, wherein the controlunit is arranged to restore the brake set speed to the first brake setspeed when the control unit has detected that the downhill condition hasended.
 10. Vehicle speed control system according to claim 7, whereinthe control unit is arranged to restore the brake set speed to the firstbrake set speed when the driver issues a resume command.
 11. A computercomprising a computer program or performing the steps of claim 1 forcontrolling vehicle speed in a vehicle equipped with brake cruisecontrol when the program is run on the computer.
 12. A non-transitorycomputer readable medium carrying a computer program comprising programcode means for performing the steps of claim 1 for controlling vehiclespeed in a vehicle equipped with brake cruise control when the programproduct is run on a computer.
 13. A control unit for controlling vehiclespeed in a vehicle equipped with brake cruise control, the control unitbeing configured to perform the steps of the method according to claim1.